Aug. 27, 2012 -- Heavy, long-term
marijuana use beginning in the teens can possibly lead to lower IQ, a provocative new study shows.

Frequent, continued use of marijuana starting before age 18 was associated with an eight-point decline in IQ in the study. The decline was not seen in users who started smoking pot in early adulthood or later in life.

The findings suggest that long-term marijuana use can cause long-term harm to some thinking abilities, such as intelligence, memory, and attention span, and that teens are uniquely vulnerable, researchers say.

"Many people today, especially young people, believe that marijuana is risk free, but this research tells us that this is not the case," says Temple University professor of psychology Laurence Steinberg, PhD, who was not involved with the study.

Pot Users, Others Followed From Birth

The research included slightly more than 1,000 adult participants in a New Zealand health study who were followed from birth.

As part of the study, IQ tests were performed at age 7, 9, 11, 13, and 38.

For the latest analysis, Duke University postdoctoral researcher Madeline H. Meier, PhD, and colleagues compared IQ scores at age 13 -- before most of the participants had used marijuana -- to those at age 38, when many had experience with the drug.

In addition to the IQ tests, family members or close friends completed questionnaires when the participants were 38 years old designed to determine if the study enrollees had trouble with attention, memory, or social functioning.

Early, Long-Term Users Had IQ Drop

People who began using marijuana in their teens and continued to use the drug for many years had an average decline in IQ of eight points by the time they reached 38 years old.

The decline could not be explained by the use of alcohol or other drugs or by having less education, and the effect was not fully reversed in those who stopped using marijuana.

The questionnaires also revealed that early, frequent marijuana users were more likely than other participants to have problems with memory and attention that impacted their daily lives.

Meier tells WebMD that marijuana use may be particularly neurotoxic to teens because adolescence is a time when the brain is maturing.

The study appears today in the journal PNAS Early Edition.

Findings Explain 'Stoners'?

Addiction researcher Wayne Hall, PhD, of the University of Queensland in Australia, says the study is among the first to assess IQ before and after the onset of marijuana use.

"This strengthens the argument against adolescents using this drug," he says.

Professor of psychiatric research Robin Murray, MD, of Kings College in the U.K., says additional research is needed to confirm the findings.

"Although one should never by convinced by a single study, I take the findings very seriously," he says.

Confirming that early, frequent marijuana use can cause lasting harm would have major implications for health policy, he adds.

"It is, of course, part of folklore among young people that some heavy users of cannabis -- my daughter calls them stoners -- seem to gradually lose their abilities and end up achieving much less than one would have anticipated," he says. "This study provides one explanation as to why this may be the case."